1. Field of the Invention and Related Art
The present invention relates to a novel toner for developing an electrostatic charge image suitable for use in image-forming processes such as electrophotography, electrostatic recording and so forth.
In general, an electrophotographic process employs a photoconductive member of a photoconductive material such as cadmium sulfide, polyvinyl carbazole, selenium, zinc oxide, silicon and the like. Electrostatic images are formed on the photoconductive members by making use of the photoconductivity of such a photoconductive material. A typical electrophotographic process, for example, includes the steps of uniformly charging a photoconductive layer (referred to as "photosensitive member" hereinafter), conducting an image exposure so as to form an electrostatic latent image on the photosensitive member, developing the electrostatic latent image with a toner powder charged in a polarity opposite to or the same as the polarity of the electrostatic latent image, and, as required, transferring the developed image to a transfer sheet. When the process employs the step of transfering the image, any residual toner on the photosensitive member which has not been transferred to the transfer sheet, is removed to clean the photosensitive member so as to enable the photosensitive member to be used repeatedly.
The removal of the residual toner is usually conducted by bringing a cleaning member or means such as a blade, fur brush or a magnetic brush into contact with the photosensitive member In this case, since the cleaning member is pressed onto the photosensitive member at a suitable level of contact pressure, problems such as damaging of the surface of the photosensitive member and deposition of toner on the photosensitive member are often experienced as a result of repeated use of the apparatus. In order to avoid deposing toner on the surface of the photosensitive member, Japanese Patent Application Laid-Open No. 48-7345 discloses an approach in which both a friction-reducing substance and a polishing material are added to a developing agent. This approach, however, suffers from the following disadvantage although it is effective in preventing deposition of toner.
The addition of the friction-reducing substance in an amount sufficient for avoiding deposition of toner makes it difficult to remove substances having low electrical resistance, e.g., paper dust, ozone-containing substances and so forth, which are generated on or attached to the surface of the photosensitive member The presence of such substances of low electrical resistance on the surface of the photosensitive member seriously impairs the quality of the electrostatic latent image on the photosensitive member, particularly when the humidity of the ambient air is high. The amounts of the friction-reducing substance and polishing substance have to be determined carefully, making it difficult to obtain a toner or developing agent which exhibits a stable performance.
Japanese Patent Application Laid-Open No. 61-59454 discloses how to suppress grinding of the photosensitive member by suitably adjusting the coefficient of kinetic friction of the toner. It is suggested that an appreciable effect is obtained by adding polyalkylene to the toner. This method does not employ any other lubricant and, hence, is superior in anti-fogging performance and image density stability. This method, however, still suffers from the following problems.
For example, polyalkylene exhibits only a small compatibility with binding resin, so that polyalkylene cannot be dispersed in the form of sufficiently small segments even through treatments such as heating and kneading. Consequently, it is often experienced that polyalkylene components come off the particles formed by a subsequent pulverizing process The separated polyalkylene exists as free particles among the toner particles and such free particles are charged in a manner different from that of the toner particles, often resulting in degradation of the image such as by fogging. Ordinary processes for preparing a toner employ a step for regulating the particle size. This is usually conducted by classification procedures through which toner particles are classified into fine particles which are too fine to be used, moderate size particles which are usually quite suitable for use and coarse particles. The fine particles are usually obtained in an amount of 10 to 40 wt % of the total weight of the toner particles. The fine particles and coarse particles are returned to the initial step of the toner preparation process so as to be mixed with the starting material for repeated use.
The above-mentioned free particles of polyalkylene are mostly classified into fine particles. This tendency is marked, particularly in magnetic toners, due to difference in specific gravity. Repeated re-use of the fine particles rich in the polyalkylene particles causes a change in the toner composition, making it difficult to obtain a uniform toner having stable developing characteristics.